Rat Spinal Cord: We propose to determine an iso-effect curve for paralysis due to irradiation of rat spinal cord since it is thought that this tussue is nonproliferative and dose limiting in radiotherapy. Ultimately, we may need to expose the animal to 30 or more dose fractions but initially we shall use up to 16 fractions and determine the dose that results in paralysis of 50% of the animal within 12 months of irradiation. Hematopoietic Stem Cell Response to Multiple Fractions: Using the endogenous spleen assay we will determine an iso-effect curve for irradiation of these cells with various multifraction radiation schemes. Since the shoulder of the survival curve for these cells is narrow we expect a different response from that of intestinal mucosa. Intestinal Mucosa: Regeneration is so effective in the jejunum that multifractionated irradiation at one-hour intervals will be tried to determine at what size dose per fraction no further sparing occurs from additional dose fractionation. Lung: The factors involved in radiation pneumonitis will be investigated. Preliminary experiments suggest that collagen deposition is not a factor. Multifraction Response of Fibrosarcoma and Mammary Carcinoma: These responses will be assayed for comparison with similar data for normal tissues. We shall try to develop a lung colony method that will permit us to use lower doses per fraction than is possible with the TCD50 technique.